There are many benefits of thin airfoils in gas turbine engine blisk fans and compressor rotors. However, when airfoils are thinned the vibratory response strength can have potentially deleterious effects on the engine or aircraft. Such effects can be reduced by mistuning the rotor. That is, shifting the natural frequencies of some airfoils within the same rotor. Achieving and maintaining the level of mistuning can ensure proper operation of the engine. One of the challenges associated with mistuning is determining and monitoring the actual level of mistuning.
To control the level of rotor mistuning in a product, both the nominal design level and measurement capabilities play key roles. Typically, the nominal design level of mistuning is set to accommodate manufacturing and anticipated field operation variations in geometry and the resulting mistuning level. This can result in significant airfoil-to-airfoil frequency variation, which can have negative impacts on performance. To produce airfoils to these specifications typically relies on sophisticated manufacturing processes.
Currently, the rotor mistuning measurement process is a tedious bench test that inherently has a large amount of variation and therefore a potential risk of inaccurately predicting the mistune of a rotor. In addition, it is labor intensive and highly dependent on the skill of the technician testing the rotor. Also, the bench test process requires that the part be taken to a lab for testing. Moreover, there is no easy to use process to check the level of mistuning in the field.
In addition to intentional mistuning of rotor and fan airfoils, airfoils have allowable repair limits set to ensure consistent operation of the entire population. If the specific geometry of the airfoil is known an increased level of allowable repair can be used. This requires a process to measure the airfoil geometry and analyze the specific repair. It's desirable that this measurement and repair be made in the field, and for example, on-wing in order to minimize cost and down time.